Vitamin D in the form of either ergocalciferol (vitamin D.sub.2) or cholecalciferol (vitamin D.sub.3) has long been added to dairy products and infant formula to assure adequate nutritional supply. These foods vary in water content from a few percent as powders to about 90% or greater when normally consumed. Most of them are of near neutral pH, although the pH of specialized formulas can vary. Milk-like infant formulas made with isolated soy protein or protein hydrolysates are also typically fortified with amounts of vitamin D similar to commercial whole milk. Some of these products, especially those made with hydrolyzed protein, are in an acidic pH range.
Although it is known that thermal or retort processing of nutritionally complete compositions often results in a substantial, e.g. 20% loss in vitamin D activity (see M. Rechcigl, Jr., Ed., Handbook of Nutritive Value of Processed Food, Vol. I, CRC Series in Nutrition and Food, page 387 (1982)), storage stability after processing has been perceived in the industry to be quite satisfactory.
The official assay for vitamin D in food substances is the rat bioassay with its substantial intra and inter assay variation. Using this assay over the years, vitamin D in these substances has appeared to be stable for considerable periods of time, even without refrigeration. For example, liquid infant formula products often have shelf lives of 12-18 months without any perceived significant loss of vitamin D activity.
We have discerned that vitamin D is much less stable upon storage in certain types of nutritional foods than previously thought. This stability problem has been solved through the use of certain esters of vitamin D in place of non-esterified forms of the vitamin.
It is taught in the art that vitamin D is slowly destroyed in an alkaline medium or in the presence of light and air, and that it is stable at a mid-pH (M. Rechcigl, Jr., Ed., Handbook of Nutritive Value of Processed Food, Vol. I, CRC Series in Nutrition and Food (1982)). Additionally, it has been taught that vitamin D is stable in corn oil, propylene glycol and milk, although vitamin D.sub.2 was shown to deteriorate in propylene glycol when diluted in water (Tractor Jitco, Inc.: Scientific literature reviews on generally recognized as safe (GRAS) food ingredients--vitamin D, PB-234 901, U.S. Dept. Commerce, July 1974; Huber, W. and Barlow, O. W.: Chemical and biological stability of crystalline vitamins D.sub.2 and D.sub.3 and their derivatives, J. Biol. Chem., 149:125-137 (1943)). It has also been disclosed that vitamin D is unstable in the presence of minerals (U.S. Pat. No. 2,758,923) and in an aqueous environment (Fraser, D. R. and Kodicek, E.: Enzyme Studies on the Esterification of Vitamin D in Rat Tissues. Biochem. J. 109:457 (1968 ).
Vitamin D esters are known to be synthesized in vivo in rats (see, for example, D. R. Fraser et al., Biochem J., 106, pp 491-496 (1986)) and have been administered to rats, chickens, and Japanese quail (see W. A. Rambech, et al. Internat. J. Vit. Nutr. Res., 51, pp 353-358 (1981) and D. R. Fraser, et al., Br. J. Nutr., 23, pp 135-140 (1969)). Heretofore addition of vitamin D esters to certain nutritional compositions to increase the stability of the vitamin has been unknown.